Field of the Invention
The present invention relates to a wiring structure including a wiring of a transparent conductive film, which is disposed, for example, on a display panel of a liquid crystal display device or the like.
Description of the Background Art
Recently, a fringe field switching (FFS) mode has been increasingly employed in a liquid crystal display device, as a liquid crystal mode capable of achieving both characteristics of a wide viewing angle and high transmittance. An FFS mode TFT array substrate includes a structure in which two transparent conductive films overlap with each other with an insulating film interposed therebetween, as shown in Japanese Patent No. 3826217, for example. On the other hand, a general twisted nematic (TN) mode TFT array substrate has one transparent conductive film. Accordingly, the number of photoengraving steps required for manufacturing the FFS mode TFT array substrate is increased by at least one step compared to that of the general TN mode TFT array substrate.
As measures against such increase in the number of steps, Japanese Patent Application No. 2010-191410 discloses a technique of reducing the number of required photoengraving steps by devising the arrangement of a transparent conductive film pattern provided on the FFS mode TFT array substrate. The configuration of the FFS mode TFT array substrate disclosed in Japanese Patent Application No. 2010-191410 can be formed through the same number of photoengraving steps as that used in manufacturing the general TN mode TFT array substrate.
As described above, the FFS mode TFT array substrate includes the structure in which the two transparent conductive films overlap with each other with the insulating film interposed therebetween. In this case, a balance between stress generated in the lower transparent conductive film and stress generated in the insulating film thereabove is not kept, and this could cause a phenomenon called “film floating” or “film peeling” (hereinafter, collectively referred to as “film floating”) in which the insulating film serving as the upper layer is peeled at an end of the transparent conductive film, in some cases.
The frequency of occurrence of this film floating is associated with a pattern density and a shape of the transparent conductive film, and is increased in a region where a pattern density of the transparent conductive film is relatively low, such as a frame region provided on the outside of a display region on the TFT array substrate, that is, in an external connection terminal part or a wiring conversion part. The inventor of the present invention has confirmed that the film floating of the insulating film on the transparent conductive film is likely to occur especially at a corner part of a pattern of the transparent conductive film arranged so as to make direct contact with the lower insulating film. Since a function of the insulating film as a protective film is lost in the portion where the film floating of the insulating film has occurred, the film floating leads to a reduction in corrosion resistance of an electrode, or occurrence of dielectric breakdown, which causes a reduction in manufacture yield of the TFT array substrate and a reduction in reliability of the TFT array substrate. Accordingly, in order to obtain a TFT array substrate having a high yield and high reliability, it is effective to take measures against the film floating of the insulating film.
In order to make the number of photoengraving steps of the FFS mode TFT array substrate equal to that of the general TN mode TFT array substrate, Japanese Patent Application No. 2010-191410 employs a configuration in which a transparent conductive film is arranged on a metal film pattern serving as a source wiring, with no insulating film interposed therebetween. In this configuration, since an area of the transparent conductive film pattern is large on the TFT array substrate, the frequency of occurrence of the film floating of the insulating film could be increased, so that the measures against the film floating becomes more important.
Moreover, according to the TFT array substrate of Japanese Patent Application No. 2010-191410, since the metal film and the transparent conductive film are required to function as the electrically same electrode or wiring, electrical connectivity between the metal film and the transparent conductive film is important. For example, preferable electrical connectivity cannot be easily obtained between a transparent electrode made of indium tin oxide (ITO) and aluminum (Al), so that a problem arises when the ITO transparent conductive film is required to be disposed on a laminated film having an uppermost layer made of an Al-based metal. The inventor of the present invention has confirmed that in the liquid crystal display device employing the TFT array substrate having the wiring structure in which ITO is arranged on Al, there is an increase in resistance in the wiring conversion part for connecting wirings of different layers of the TFT array substrate, which causes a display defect such as a line defect.
On the other hand, since an Al-based thin film has a low resistance value, the Al-based thin film is increasingly employed in a signal wiring of the TFT array substrate along with an increase in resolution and an increase in size of a screen of the liquid crystal display device. Therefore, in view of the development of the FFS mode TFT array substrate, it is important to improve the electrical connectivity between the transparent conductive film and the metal film, particularly, between the transparent conductive film and the laminated film having the Al-based thin film serving as its upper surface.